1. Field of the Invention
The present invention relates to a real index guided semiconductor laser device capable of high power operation that can be used preferably in communications, laser printers, laser medical treatment, laser processing and the like.
2. Description of the Related Art
FIGS. 4A to 4C are cross-sectional views showing the structure of a real index guided semiconductor laser device (hereinafter, referred to as xe2x80x9cDCH-SAS type LDxe2x80x9d) having a decoupled confinement heterostructure as shown in Japanese Unexamined Patent Publication JP-A 11-154775 (1999) and a method for manufacturing the same.
In FIG. 4A, an n-type AlGaAs cladding layer 2, an n-type AlGaAs optical waveguide layer 3, an n-type AlGaAs carrier blocking layer 4, GaAs/AlGaAs quantum well active layer 5, a p-type AlGaAs carrier blocking layer 6, a part of a p-type AlGaAs optical waveguide layer 7 are formed in this order by crystal growth on an n-type GaAs substrate 1. Then, as shown in FIG. 4B, an SiO2 stripe mask 8 is formed on the grown epitaxial substrate, more specifically, on a predetermined region on the p-type AlGaAs optical waveguide layer 7a by evaporation and photolithography techniques. Then, as shown in FIG. 4B, an n-type AlGaAs refractive index control layer 9 is formed by selective growth on the region other than the region where the SiO2 stripe mask 8 is formed. Then, after the SiO2 stripe mask 8 is removed, as shown in FIG. 4C, a p-type AlGaAs optical waveguide layer 7b that is the rest of the optical waveguide layer, a p-type AlGaAs cladding layer 10, and a p-type HGaAs contact layer 11 are formed in this order by crystal growth. Thus, a DCH-SAS type LD is manufactured. The p-type AlGaAs optical waveguide layer 7a and the p-type AlGaAs optical waveguide layer 7b constitute one optical waveguide layer 7.
In such a DCH-SAS type LD, a semiconductor material having a lower refractive index than that of the optical waveguide layer 7 is buried as the refractive index control layer 9. This creates an effective refractive index difference also in the direction parallel to the active layer 5 in a striped region R1 (which may be referred to as a xe2x80x9cwindowxe2x80x9d in the following) in which the refractive index control layer 9 is not formed in the optical waveguide layer 7 (the direction parallel to the active layer 5 is the width direction of a striped window R1). Thus, laser light is confined also in the width direction of the striped window R1, so that single transverse mode oscillation can be obtained highly efficiently at a low threshold.
Furthermore, in the production method employing selective growth as shown in FIGS. 4A to 4C, an etching process in which processing precision is low is eliminated, and the refractive index control layer 9 can be formed utilizing the high control properties of the crystal growth method such as MOCVD, MOMBE, and MBE.
In general, the crystal growth technique has high control properties. However, immediately after the start of growth on a substrate that has been exposed to air, growth is specifically unstable. For example, immediately after growth starts, the growth rate is reduced, or in the worst case, an idle running time during which no growth is caused occurs. In the method for manufacturing a semiconductor laser device employing selective growth as described above, the refractive index control layer 9 is grown directly on the epitaxial substrate that has been exposed to air. Therefore, the thickness of the refractive index control layer 9 is unstable due to the occurrence of the idle running time or the like. This causes the problem that the reproducibility of the effective refractive index difference in the width direction of the striped window R1 is not good. In particular, when the refractive index control layer 9 is designed to be thin, this problem is more serious.
It is an object of the invention to provide a semiconductor laser device comprising a refractive index control layer which can be formed by selective growth under highly controlling the thickness thereof, and having an effective refractive index difference of good reproducibility, and a high production yield.
The present invention provides a real index guided semiconductor laser device comprising:
an active layer;
an optical waveguide layer provided at least on one side of the active layer, the optical waveguide layer having a band gap energy equal to or more than a band gap energy of the active layer;
a cladding layer provided on an outer side of the optical waveguide layer, the cladding layer having a band gap energy equal to or more than the band gap energy of the optical waveguide layer;
a refractive index control layer having a striped window, buried in the optical waveguide layer or buried between the optical waveguide layer and the cladding layer, the refractive index control layer being formed by selective growth; and
a semiconductor layer being formed by selective growth prior to the formation of the refractive index control layer by the selective growth,
wherein a material of the semiconductor layer is selected so that, in a laminated portion including the semiconductor layer and the refractive index control layer, a change in effective refractive index due to a change in thickness of the semiconductor layer is smaller than a change in effective refractive index due to a change in thickness of the refractive index control layer.
According to the invention, the semiconductor layer is selectively grown prior to the refractive index control layer. Therefore, the selective growth is stabilized during the growth of the semiconductor layer, the refractive index control layer can be formed by selective growth under highly controlling the thickness of the refractive index control layer. In the laminated portion including the semiconductor layer and the refractive index control layer, the change in effective refractive index due to a change in thickness of the semiconductor layer is smaller than that of the refractive index control layer. Therefore, even if the thickness of the semiconductor layer is reduced due to an occurrence of idle running time or the like, the influence on the effective refractive index in the laminated portion can be suppressed to be smaller than in the case where the semiconductor layer is not used. Consequently, the variation of the difference in effective refractive index between two laminated portions including the semiconductor layer and the refractive index control layer, and a laminated portion including a window sandwiched between the two laminated portions becomes smaller among semiconductor laser devices.
Thus, the refractive index control layer of the semiconductor laser device can be formed by selective growth under highly controlling the thickness thereof, and a desired effective refractive index difference, which is a difference in effective refractive index between the laminated portions including the semiconductor layer and the refractive index control layer, and a laminated portion including the window of the refractive index control layer, can be created with high reproducibility, with the result that an improved production yield can be realized.
In the invention, it is preferable that a change in effective refractive index difference due to a change in thickness of the semiconductor layer is 5xc3x9710xe2x88x926/nm or less, wherein the effective refractive index difference is a difference in effective refractive index between the laminated portions including the semiconductor layer and the refractive index control layer, and a laminated portion including the window of the refractive index control layer.
According to the invention, the effect of reducing the effective refractive index due to the formation of the semiconductor layer having a low refractive index in the optical waveguide layer is substantially offset by the effect of increasing the effective refractive index due to an increase of the entire thickness of the optical waveguide layer. The crystal growth is stabilized with growth corresponding to 10 nm to 50 nm. Therefore, if the change in effective refractive index due to a change in thickness of the semiconductor layer is designed to be 5xc3x9710xe2x88x926/nm or less, the effective refractive index difference is controlled by the refractive index control layer formed to have a desired thickness, substantially without being affected by the thickness of the semiconductor layer. Consequently, the refractive index control layer of the semiconductor laser device can be formed by selective growth under highly controlling the thickness thereof, and a desired effective refractive index difference, which is a difference in effective refractive index between the laminated portions including the semiconductor layer and the refractive index control layer, and a laminated portion including the window of the refractive index control layer, can be created with high reproducibility, with the result that an improved production yield can be realized.
In the invention, it is preferable that a thickness of the refractive index control layer is 300 nm or less in terms of further effectiveness.
According to the invention, even if a refractive index control layer is formed to be thin and an occurrence of idle running time or the like affects the effective refractive index difference significantly, a desired thickness thereof can be reproduced precisely. Therefore, the ability of controlling the thickness of the refractive index control layer formed by selective growth can be increased, a desired effective refractive index difference can be created with good reproducibility, and the production yield can be improved.
According to the invention, it is possible to control the effective refractive index difference in the width direction of the window with good reproducibility by stabilizing crystal growth during growth of the semiconductor layer whose influence on the effective refractive index is suppressed to be small to improve the ability of controlling the thickness of the subsequently grown refractive index control layer.